Element fabrication substrate

ABSTRACT

A substrate used for fabricating devices thereon includes an insulating film, and a monocrystal Ge thin layer formed on the insulating film in contact therewith, the monocrystal Ge thin layer having a thickness not more than 6 nm. The monocrystal Ge thin layer has a thickness not less than 2 nm and a compressive strain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2003-374571, filed on Nov. 4,2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a substrate for device fabrication, thesubstrate having a monocrystal Ge thin layer for forming field effecttransistors of high-performance thereon, a semiconductor device usingthis substrate, and a method for manufacturing the substrate.

Conventionally, a method for increasing a drive current per a unit gatelength by shortening a gate length of an individual transistor andthinning a gate insulation layer is adopted for realizinghigh-performance/high function of CMOS circuit device. By this method,the size of a transistor to provide a necessary drive current isdecreased. This makes it possible to realize a high integration, and tolower a drive voltage, resulting in decreasing a power consumption per aunit element.

However, improvement of performance required in late years increases atechnical barrier to be solved for the purpose of decreasing a gatelength. It is effective to use channel materials of high mobility inorder to relax the circumstances. Ge is an influential candidate for thechannel materials. Ge has higher mobility than Si with respect toelectrons and holes. It is known that hole mobility largely increases bygiving a compressive strain to Ge. In a bulk semi-conductor, holemobility is low in comparison with electron mobility. Therefore,increase of hole mobility contributes to higher performance of acircuitry.

There is a problem that a parasitic capacitance of source and drainjunctions disturbs a transistor operation which is caused bymicronization of a transistor. A fully-depleted type device structurewherein a buried insulator layer is formed under a semi-conductor thinchannel layer is considered in order to avoid this problem. The filmthickness of the semi-conductor thin channel layer in this case is notmore than about 6 nm with respect to a transistor of a gate length 25nm, for example. If a channel is formed by a strained Ge thin film on aburied insulating layer combining the feature of a strained Ge channeland that of a fully-depleted type device structure, it is possible tofabricate a high performance transistor. However, an on-insulating filmlaminating strain Ge thin layer having these both features is notrealized under the present circumstances.

In a document “T. Tezuka, N. Sugiyama, S. Takahi, Appl. Phys. Lett. 79,p1798 (2001)”, the inventors of the present invention proposes theGe-condensation by oxidation method to make Ge composition in SiGeincrease by oxidizing a monocrystal Si layer formed on an insulatingfilm on a supporting substrate and a monocrystal SiGe layer containingGe composition of about 10% which is formed on the Si layer. However,this method is a method for manufacturing a lattice-relaxed SiGe layerof high Ge composition as a substrate for a strained Si layer, unlike amethod for forming a strained Ge thin layer. Further, this method doesnot consider thinning the film thickness of the Ge layer.

The substrate having a strained Ge thin layer on an insulating film isexpected as a substrate used for making a field effect transistor withhigh mobility. However, a technique to form a strained Ge thin layer ofextremely thin film thickness on an insulating film has not yet beenrealized.

The present invention is to provide a substrate for device fabricationhaving an extremely thin Ge layer on an insulating film.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided asubstrate for device fabrication comprising: an insulating film; and amonocrystal Ge thin layer formed on the insulating film in contacttherewith, the layer having a thickness not more than 6 nm.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1A to 1D show sectional views of substrate structures in steps ofsubstrate manufacturing according to an embodiment of the presentinvention;

FIG. 2 shows a temperature dependence of melting Ge composition x_(m)(T)of Si_(1-x)Ge_(x); and

FIG. 3 is a sectional view of an device structure of a MOSFET using thedevice fabrication substrate of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described referring todrawings.

FIGS. 1A to 1D illustrate sectional structures in steps of manufacturinga substrate used for fabricating devices such as transistors thereon,according to an embodiment of the present invention.

As shown in FIG. 1A, a SOI substrate 10 is prepared by forming aninsulating film 12 of, for example, SiO₂ on a Si substrate 11 and thenforming a Si thin layer 13 on the insulating film.

As shown in FIG. 1B, Si_(1-x)Ge_(x) crystal is grown to a thickness ofd_(i) (nm) with Ge composition x_(i) on the Si thin film 13 on theinsulating film Si by, for example, the CVD method to form a SiGe layer15. In the present embodiment, assuming d_(i)=40 nm, and x_(i)=0.15.

As shown in FIG. 1C, the substrate is subjected to thermal oxidation inoxidation ambient atmosphere. In the step of FIG. 1C, a Si oxidationfilm 16 is formed by oxidizing only Si in the Si thin layer 13 and SiGelayer 15. In this time, Ge is rejected from the oxide film 16 andaccumulated in the SiGe layer 15, resulting in that Ge composition inthe SiGe layer 15 increases. When the Ge composition in the SiGe layer15 is not less than 60%, the oxidation temperature or heatingtemperature exceeding 1000° C. is desirable. This provides an effectthat the Ge composition in the SiGe layer 15 is uniformized andgeneration of defects is suppressed, and an effect to shorten anoxidation time. The temperature must be decreased with increase of Gecomposition. Finally, the oxidation temperature has to be not more thanthe melting temperature of Ge of 937° C.

FIG. 2 shows a relation between the Ge composition of SiGe layer and themelting temperature. It is understood from FIG. 2 that the meltingtemperature decreases as Ge composition increases according to progressof oxidation. The SiGe layer 15 should not be melted in order to remaina strain in the Ge thin layer that is finally obtained. Accordingly, itis understood that a temperature less than the melting temperature ofSiGe but sufficiently high temperature is needed for oxidation of theSiGe layer, and the final heating temperature must be not more than 937°C.

According to the experiment of the present inventors, the finaloxidation temperature must be not more than 900° C. for formation of aGe thin layer whose thickness is 15 nm. The final oxidation temperatureof 930° C. precludes formation of a high quality GOI layer having astrain. In other words, at a final oxidation temperature near themelting temperature of Ge, for example, 930° C., degradation ofcrystalline quality is recognized. However, a high quality singlecrystal was obtained at a final oxidation temperature not more than 900°C. In the case that a final film thickness of Ge layer less than 3 nm,the final oxidation temperature must be further decreased. Concretely,setting the final oxidation temperature of 850° C. for a GOI layer of athickness less than 3 nm provides a good quality crystal.

A conventional Ge condensation by oxidation method needs to set asubstrate heating temperature at a high temperature exceeding 1000° C.for the purpose of relaxing the lattice strain of the SiGe layer anduniformize a Ge composition profile. However, although the substrateheating temperature is set at a temperature exceeding 1000° C. in aninitial stage, it is important to remain as much strain as possible inthe SiGe layer in the final stage (Ge composition more than 80%).Therefore, in the present embodiment, the substrate heating temperatureexceeding 1000° C. in the initial stage is decreased gradually to 900°C. in the final stage. The composition profile is able to be uniformizedsufficiently even a low temperature not more than 900° C. (a diffusioncoefficient of Si in the Ge single crystal is sufficiently large).

In this way, in the present embodiment, for fabrication of Ge layer bythe Ge condensation by oxidation method, the oxidation temperature isset at a temperature exceeding 1000° C. at first, and the oxidation isdone at the temperature of 900° C. at last. As a result, Ge in the SiGelayer 15 is condensed, and a pure monocrystal Ge thin layer 14 (filmthickness df) is finally formed on the insulating film as shown in FIG.1D. In this way, the monocrystal SiGe layer 15 are transformed to themonocrystal Ge thin layer 14.

In the present embodiment, a monocrystal strained Ge thin film having astrain of 1.1% and df=6 nm is formed on an insulating film. Thissubstrate for device fabrication has a structure that a strained Ge thinfilm is directly in contact with a buried insulating film.

If the Ge thin layer finally formed is too thin, it is impossible togive a compression strain. According to an experiment of the inventorsof the present invention, the following became clear. That is, it isimpossible to give a compression strain if the Ge thin layer is thinnerthan 2 nm. If it is not less than 2 nm, it is possible to give acompression strain. If it is not less than 4 nm, it is possible to givea sufficient compression strain. Accordingly, the lower limit of thethickness of the Ge thin layer is 2 nm, preferably not less than 4 nm.

In a conventional method, a perfect monocrystal Ge layer on aninsulating film is formed by directly transferring a thin Ge layer onanother substrate. In this case, it is difficult to make the thicknessof the Ge layer not more than 10 nm. However, in the present embodiment,it is possible to make the thickness of the Ge layer not more than 6 nm,e.g. about 2 nm.

Using a substrate for device fabrication as shown in FIG. 1D, a gateelectrode 22 is formed via a gate insulating film 21 as shown in FIG. 3.Further, a source region 23 and a drain region 24 are formed. As aresult, a high-performance MOSFET is fabricated because of high mobilityof a strained Ge channel. It is possible to realize a fully-depletedtype device structure having a strained Ge channel by setting the filmthickness of the strained Ge thin film 14 at not more than 6 nm with agate length of 25 nm, and therefore a MOSFET of higher performance isfabricated.

The hole mobility largely increases by giving the Ge thin layer 14 acompressive strain, and a difference between the hole mobility and theelectron mobility can be reduced. This is effective when a CMOSstructure is fabricated.

The present invention is not limited to the embodiment. The thickness ofthe monocrystal Ge thin layer is not limited to the embodiment, and maybe not more than 6 nm to get performance enhancement intended by thepresent invention with respect to a device of a short gate length.Further, various conditions to make the thickness of the Ge thin filmnot less than 2 nm may be set for the Ge layer to have a sufficientstrain. In addition, it is most desirable that the monocrystal Ge thinlayer has a strain in the light of mobility. However, even if it has nostrain, it provides an enhancement effect on mobility in comparison withSi. In this case, a range of the heating temperature of the SiGe layer,the thickness of the final Ge thin layer and so on becomes wider than ina case for forming a strained Ge thin layer.

In addition, the Ge composition in the SiGe layer before heat treatmentis set to 15%. However, if the Ge density is too high, high qualitysingle crystal is not provided. Accordingly, it is desirable that Gecomposition at the time of the SiGe formation is not less than 60%.Further, a SiGe layer formation method is not limited to a CVD method,and should use a method for forming a thin SiGe layer on a Si layer inuniform and high quality.

According to the present invention, by improving an Ge condensation byoxidation method for increasing Ge composition by oxidation, a highquality monocrystal Ge thin film can be formed on an insulating film byoxidizing sufficiently a SiGe layer containing a comparatively largeamount of Ge composition at a temperature less than a meltingtemperature of SiGe.

In particular, it is possible to make a Ge thin layer having acompressive strain by making the film thickness of the final Ge thinlayer not less than about 2 nm. Fabricating MOSFET by using such a Gethin layer allows realizing a high-performance CMOS structure.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A substrate used for fabricating a device thereon, comprising: aninsulating film; and a monocrystal Ge thin layer formed on theinsulating film in contact therewith, the layer having a thickness notmore than 6 nm.
 2. The substrate according to claim 1, wherein the Gethin layer has a compressive strain.
 3. The substrate according to claim1, wherein the Ge thin layer has a thickness not less than 2 nm and acompressive strain.
 4. The substrate according to claim 1, wherein themonocrystal Ge thin layer is made of Si_(1-x)Ge_(x) formed on theinsulating film.
 5. The substrate according to claim 1, which includes aSi oxide film formed on the monocrystal Ge thin layer by oxidation. 6.The substrate according to claim 1, wherein the monocrystal Ge thinlayer contains Ge composition not less than 60%.
 7. A semiconductordevice comprising; an insulating film; a monocrystal Ge thin layerformed on the insulating film and having a compressive strain and athickness not more than 6 nm and not less than 2 nm; and a field effecttransistor using the Ge thin layer as a channel.
 8. The semiconductordevice according to claim 7, wherein the monocrystal Ge thin layer ismade of Si_(1-x)Ge_(x) formed on the insulating film.
 9. Thesemiconductor device according to claim 7, which includes a Si oxidefilm formed on the monocrystal Ge thin layer.
 10. The semiconductordevice according to claim 7, wherein the monocrystal Ge thin layercontains Ge composition not less than 60%.
 11. A method of manufacturinga substrate used for fabricating a device thereon, comprising: forming amonocrystal SiGe layer on a monocrystal Si layer formed on an insulatingfilm; and heating the monocrystal Si layer and the monocrystal SiGelayer to oxidize them for forming a Si oxide film on the monocrystalSiGe layer, and to transform the monocrystal SiGe layer to a monocrystalGe thin layer having a thickness not more than 6 nm.
 12. The methodaccording to claim 11, wherein the monocrystal SiGe layer contains Gecomposition not less than 60%, and the heating includes heating themonocrystal Si layer and the monocrystal SiGe layer at a temperature notmore than a melting point of the monocrystal SiGe layer, with setting aheating temperature to a temperature exceeding 1000° C. at a time ofoxidizing at first, and setting at a temperature not more than 900° C.at last while decreasing the heating temperature gradually, to form themonocrystal Ge thin layer having a compressive strain in a thickness notless than 4 nm.
 13. The method according to claim 11, including settingthe oxidization temperature at a temperature not more than 850° C. toform the monocrystal Ge thin layer having a compressive strain in athickness not less than 2 nm.
 14. The method according to claim 11,wherein the monocrystal Ge thin layer is made of Si_(1-x)Ge_(x) formedon the insulating film by crystal growth.
 15. The method according toclaim 11, wherein Ge composition in the monocrystal SiGe layer beforeheat treatment is set to 15%.